Fiber-reinforced composite materials are utilized in a wide range of applications, from sports and leisure-related purposes to transportation purposes such as in aircraft, and for industrial applications. Molding methods using dies are the most common molding methods for such fiber-reinforced composite materials.
Examples of such methods include the hand layup method wherein a reinforcing fiber material such as a cloth is impregnated with a resin while being laid up along a die, or a “prepreg” of a reinforcing fiber material pre-impregnated with a resin is laid up along a die, and after repeating this process, is cured and released from the die to obtain a molded article; the resin transfer molding method wherein a reinforcing fiber material such as a cloth is set in a die, injected with a resin and cured and then released from the die to obtain a molded article; and the molding compound method wherein a reinforcing fiber material cut into staple fibers is mixed with a resin to prepare a molding material which is injected into a die and then cured and released from the die to obtain a molded article.
The dies used for such molding methods are made of various materials such as metal, resin or wood. Metal dies exhibit excellent heat resistance and durability but are costly because of the effort and labor required for manufacture, while resin dies and wooden dies have the advantage of cheaper availability despite inferior heat resistance and durability.
In order to meet the increasing requirements of recent years, it has been common to employ cheaply available resin dies for the increased production of various types of molded articles on a small scale, while wooden dies are sometimes used for molding of large-size fiber-reinforced composite materials for marine shipping and the like.
Molding methods employing such resin dies or wooden dies do not permit molding at high temperature because of the inadequate heat resistance of the resin or wooden dies themselves, and therefore cannot be applied for molding of articles with high heat resistance.
As a method for obtaining molded articles made of fiber-reinforced composite materials with high heat resistance using low-heat-resistant resin or wooden molding dies, there has been proposed a method wherein a low-heat-resistant molding die is utilized for primary curing at a relatively low temperature of 100° C. or below to prepare a releasable primary cured molded article, after which the primary cured molded article is released from the low-heat-resistant molding die and subsequently allowed to stand in a high-temperature atmosphere for secondary curing, and this method has been attempted for molding of large-sized molded articles that require high heat resistance, such as articles destined for the shipping and aerospace industry.
Incidentally, when such molding methods employing curing means comprising both primary curing and secondary curing are carried out using prepregs; it is essential for the prepreg to be cured for a short time at a relatively low temperature of 100° C. or below to achieve releasable curing, to obtain a cured product with high heat resistance by secondary curing at high temperature, and to ensure that the prepreg itself has excellent stability and satisfactory handleability at room temperature.
In light of these circumstances, numerous technical documents such as Japanese Unexamined Patent Publication HEI No. 11-302412, for example, have disclosed resin compositions that are relatively stable at room temperature and curable at comparatively low temperatures of 70-100° C., and although these resin compositions are all curable at low temperature and give primary cured products with excellent mechanical properties, they do not exhibit adequate heat resistance even during the subsequent secondary curing at high temperature.
On the other hand, conventional resin compositions which give cured products with satisfactory heat resistance have also been problematic in that a long time is required for releasable primary curing at relatively low temperatures of 100° C. and below.